Carbon dioxide removal from breathable atmospheres

ABSTRACT

In the purification of breathable atmospheres, carbon dioxide is selectively removed therefrom and concentrated in a process wherein the contaminated gas stream is passed through a porous bed comprising beads of a solid porous polymer of divinylbenzene coated with polyethylenimine wherein the carbon dioxide is sorbed, the carbon dioxide subsequently being removed from the bed by vacuum desorption at low temperature in a bed regeneration sequence.

I United States Patent [151 3,659,400 Kester [4 May 2, 1972 s41 CARBONDIOXIDE REMOVAL FROM 2,323 1 329 Ulricildet al. 2223 7 Arno i BREATHABLEATMOSPHERES 3,547,684 12/1970 Hollis et al. ..55/67 X [72] Inventor:Frank L. Kester, West Granby, Conn.

' i l C l-l [73] Asslgnee gxg g g f orporauon East an PrimaryExaminer-Reuben Friedman Assistant Examiner-R. W, Burks [22] Filed: July21, 1970 Attorney-Richard N. James [21] Appl. No.: 56,794

[57] ABSTRACT [52] US. Cl. ..55/33, 23/2 S, 5555/5688, In thepurification of breathable atmospheres carbon dioxide is selectivelyremoved therefrom and concentrated in a [51] Int. Cl ..B0ld 53/02Process wherein the contaminated gas stream is passed [58] FleldofSearch ..23/25,2A, 150; 55/28, 33, through a porous bed comprisingbeads of a Solid porous 55/68 74, 387 polymer of divinylbenzene coatedwith polyethylenimine wherein the carbon dioxide is sorbed, the carbondioxide subl l References Cited sequently being removed from the bed byvacuum desorption d t v UNITED STATES PATENTS at low temperature m a heregenera ion sequence 2,818,323 l 2/1957 Haensel ..23/2 S 1 Claims, 3Drawing Figures 'fl/ebfl/f/fl/dX/flf QmVYMYQNNQMHY BACKGROUND OF THEINVENTION The present invention relates to a method for the selectiveremoval of carbon dioxide from breathable atmospheres.

In the development of closed-cycle life support systems, there is a needfor more efficient methods of removing carbon dioxide from cabin air. Atthe present time, the systems typically require close control of themoisture content of the airstream for carbon dioxide removal and, inaddition, typically require high bed temperatures for regeneration. Thisnecessitates considerable dehumidification apparatus operable inconjunction with the adsorption equipment and, further in many cases,because of the temperatures involved, considerable weight and powerlosses. Furthermore, in some instances, the temperature levels requiredare conducive to rapid bacterial multiplication.

As reported in an article titled Water Analysis by Gas ChromotographyUsing Porous Polymer Columns, Journal of Gas Chromotography, July 1966,it was observed that columns packed with porous aromatic hydrocarbonpolymers can separate the common gases for water analysis. In particularthe polymer of divinylbenzene was found-to be effective. Furthermore, itwas found that modification of the polymer with polyethylenimine waseffective for the separation of the amine gases. Other investigatorshave also researched the problem as reported in the publication titledSeparation of Gaseous Mixtures Using Porous Polyaromatic Polymer Beads,Analytical Chemistry, Vol. 38, No. 2., February 1966.

What is still required in the lift support technology is a simple,reliable method for selectively separating carbon dioxide from thenon-contaminating components of a breathable airstream in an efficientmanner.

SUMMARY OF THE INVENTION The primary objective of the present inventionis to provide a process for the selective removal of carbon dioxide frombreathable atmospheres in a reliable, efficient manner.

Briefly described, the process contemplates:

circulating the contaminated breathable gas containing carbon dioxidethrough a porous bed comprising beads of a solid porous polymer ofdivinylbenzene coated with polyethylenimine; v

sorbing carbon dioxide in the bed while concomitantly desorbing andsorbing water vapor, if present, in the carbon dioxide sorption cycle;

periodically isolating the bed from the breathable gas stream andselectively desorbing the carbon dioxide therefrom;

and subsequently reusing the desorbed bed for further decontamination ofthe breathable gas.

In the process, carbon dioxide may be sorbed in either a wet or dry bed,or from a humid or dry gas stream, eliminating the need for priorancillary moisture control apparatus, and carbon dioxide removal from ahumid gas stream is selective with little or no water loss. Furthermore,regeneration of the bed in the carbon dioxide concentration cycle isperformed at relatively low temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simple schematic of acarbon dioxide concentrator subsystem.

FIG. 2 is a graphical representation of the sorption-desorption processof the present invention.

FIG. 3 is a graph similar to that of FIG. 2 showing the characteristicsof the process at various operating parameters.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As previously mentioned, thepresent process contemplates the selective removal of carbon dioxidefrom breathable gases in regenerable porous beds. Thus, the individualbed is cycled through a sorption sequence wherein carbon dioxide iswherein the carbon dioxide is selectively purged from the bed duringregeneration thereof; This may be accomplished in a very simplesubsystem such as that illustrated in FIG. 1.

The mechanism by which the polyethylenimine coated divinylbenzene bedeffects removal of the carbon dioxide is not completely understood. Theprocess may involve chemisorption, adsorption or permeation, or perhapssome other mechanism. As described herein, the process or processesinvolved are collectively referred to as sorption.

When processed in the manner described, at the beginning of the sorptioncycle, water vapor present in the bed, if any, typically from a previoussorption cycle, is flushed ofi the bed and returned to the gas stream.Part way through the sorption cycle, the water has essentially all beenflushed oi? the bed into the gas stream and the bed begins to resorbwater vapor, this time from the incoming gas stream. At the same timecarbon dioxide is being sorbed in the bed. Thus, in the presence ofwater vapor, as is usually the case in the gas stream of a life supportsystem, water is both desorbed and sorbed in the bed in the carbondioxide sorption cycle under the proper conditions. Also with properoperation, carbon dioxide is subsequently vacuum desorbed from the bedat low temperature, such as F. and 1 p.s.i.a., with little or nor waterloss in the sequence. Thus, in the complete cycle, the breathableatmosphere is purged of carbon dioxide which in turn is concentratedwith little or no water vapor contamination.

In one series of tests solid beads of divinylbenzene (available asPORAPAK Q from Waters Associates, Inc. Framingham, Mass) in a mesh sizeof 5080 were coated with polyethylenimine to 10 percent by weight andpacked in a bed. Gas system conditions were as follows:

Sorption Gas Supply 1% C0 in N Part. Press. Co 7.6 mm Hg Total Press.760 mm Hg (14.7 p.s.i.a.) Bed Temp. 72F. Gas Flow I00 cc./min. Dew Point50 F. Desorption Bed Temp. 180 F. Press. 0.53 p.s.i.a.

Generally speaking, the following results were attained. The maximumcapacity of the bed in terms of carbon dioxide capture appeared to occurat lineal gas velocities of about 0.01 ft./sec. At 1 p.s.i.a., the bedcapacity for carbon dioxide was about 1.45 percent by weight, althoughthe dynamic capacity was seen to vary with desorption conditions. Whenthe bed was subjected to very hard vacuum conditions (=200 microns) indesorption, a bed capacity maximum of about 3 weight percent wasobserved. Decreasing the desorption pressure may thus be seen toincrease the potential carbon dioxide capture capacity. However, at thishard vacuum and below about I p.s.i.a. significant quantities of waterare also desorbed. Thus, a carbon dioxide concentrator operated in thismanner would require carbon dioxide-water separation.

In related studies, the extent of carbon dioxide desorption from the bedunder differing conditions of pressure was investigated. The results aresummarized in Table I.

During bed regeneration, the efiect of temperature was alsoinvestigated. As expected, it was found that an increase in temperaturedid effect an increase in effective desorption pressure. Thus, at 150 F.the desorption pressure was found In Table III, Design 1 represents thesituation where thermal energy is obtained through Joule heating andDeisgn 2 represents an advanced concept where thermal energy arises fromisotopic heat. In terms of performance and reliability, all

to be just about twice that at 100 F. However, a principal ad- 5 of thesystems are relatively equal. The system required by the vantage of thepresent process is that high temperatures are Present Rimless A J y aSignificant advantage fiver those of not required for eflectiveprocessing of the gas stream and, B and C n terms o both welght andvolume- Whfle process A hence, the entire cycle may be conductedisothermally. does not j y a slgmficam Weigh? advantage Over It Shouldbe noted that process D requires a number of conden- To ascertain therelative effectiveness and efficiency of the sor/separators operating atabout 200 F, Th units present process, it was compared with severalalternative e nt a prime location for bacterial propagation and, schemeswhich have previously been proposed. In this study, f h th o d o e amapparatus, when the parameters were all compared to the nearest commonrequired, necessitates precise pressure regulation for proper baseline.For example, in computing relative weights, not only operation.

TABLE III Process A B c 1) Design 1 2 1 2 1 2 1 2 Parameter equiv. wt.,1115.:

Basic unit; 257 257 335 303 337 337 2611 231 s ares 295 295 331 331 303303 231 231 Elee. power..... 521 86 376 237 865 142 453 81 Thermalpower, 48 15 8O fr. Rod. load 67 67 114 114 263 203 105 1115 Total 1,140753 1,206 1,090 1,768 1,165 1.14s 300 Volume (it. )t 28 37 33 24 sorbentweight was considered, but necessary system, redundencies, and weightpenalties arising from energy requirement directly attributable to theparticular process involved.

The comparisons were based on the following basic criteria: Endurance500 days no resupply Co Production 0.81 lb. Co /hr.

System Press. 7.510 p.s.i.a.

-Co Press. 7.6 mm. Hg.

TABLE II is it possible to Process A D Operate bed dry yes no no Operateat low humidity yes no no Operate at high humidity yes no no Thus, therehas been provided a process for selectively removing carbon dioxide frombreathable atmospheres which is not only efficient, but reliable andlightweight. While the invention has been described in connection withcertain preferred embodiments for the purposes of illustration, thoseskilled in the art will recognize that the invention is its broaderaspects is not limited to the specific details described, and departuresfrom such details may be made within the scope of the appended claimswithout departing from the principles of the invention and withoutsacrificing its chief advantages.

What is claimed is:

l. The method of selectively removing carbon dioxide from breathableatmospheres which comprises:

circulating the breathable gas containing carbon dioxide through aporous bed comprising finely divided particles of polymerizeddivinylbenzene coated with 01 ethyleniminea sor mg carbon dioxlde 1n thebed at a temperature of about 50l00 F. while concomitantly desorbing andsorbing water vapor, if present, in the carbon dioxide adsorption cycle;

periodically isolating the bed from the breathable gas stream;

selectively desorbing the carbon dioxide from the bed at a pressure of1-3 p.s.i.a. at a temperature not exceeding about 200 F.; and

subsequently reconnecting the bed to the breathable gas stream forfurther carbon dioxide removal therefrom.

